1. Technical Field
This invention relates generally to a fault detection scheme and, more particularly, to a fault detection and correction circuit for detecting short circuit faults on a magnetic or variable reluctance sensor that produces an alternating differential voltage in response to rotation of a wheel.
2. Discussion
Inductive magnetic sensors are commonly employed for automotive applications and the like to provide timing signals which enable the determination of position and speed of a rotating wheel. For example, specific applications may include the determination of engine crankshaft position and speed (i.e., RPM) or the determination of wheel speed for anti-lock braking systems. Inductive magnetic sensors for these types of applications are commonly referred to as variable reluctance sensors.
The variable reluctance sensor is generally located adjacent to a rotating wheel which typically has a plurality of circumferentially spaced slots formed therein. The sensor has an inductive magnetic pick-up that is generally made up of a pick-up coil wound on a permanent magnetic core. As the wheel rotates relative to the pick-up coil, an alternating voltage is generated on the pick-up coil when the slots in the wheel travel past the sensor. The frequency of the altenating voltage is then determined to achieve rotational speed information about the wheel.
The alternating voltage that is produced with the variable reluctance sensor has peak voltages that generally vary in amplitude according to the rotational speed of the wheel. The voltage signal is commonly translated to a 0-5 volt pulse train which in turn provides the frequency information to a control or processing module. Many systems, such as the anti-lock braking systems, perform safety related functions and therefore may require fault detection and diagnostics so that a user may be warned of possible sensor failures.
A number of faults can be detected on the sensor leads by monitoring the sensor outputs for a short circuit. A short circuit may distort amplitude, DC bias and phase of a differential signal which can cause inaccuracies in frequency determination. This in turn may lead to errors in rotational speed and position information. There are a number of single-ended sensor configurations in which short circuit detection is generally not available. This is because typical single-ended systems would require the addition of an offset voltage reference which tends to add unacceptable variations to a given system.
With a differential voltage sensor configuration, conventional fault detection could be provided by detecting a shift in the DC bias. A number of fault detection circuits exist for detecting such a short circuit fault. However, existing systems generally only inform the user or provide a system shutdown in response to such fault detection. These systems generally do not provide a means of altering the system so as to allow for continued operation thereof.
One of the objects of the present invention is to provide an enhanced fault detection circuit that is capable of detecting a fault with a differential voltage that is generated by a variable reluctance sensor.
Another object of this invention is to provide a fault detection circuit that detects a short circuit to ground and a short circuit to power sources for a differential voltage and further adapts the system so as to correct for any short circuit and thereby allow for continued operation of a system.
Still another more specific object of this invention is to provide for such a fault detection circuit which detects short circuits associated with the differential leads of a variable reluctance sensor and may modify the differential voltage so as to correct for errors which may otherwise be associated with a short circuit.